1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having good image quality.
2. Discussion of the Related Art
Recently, a thin film transistor liquid crystal display device(TFT LCD) has been used as a display device for portable televisions and notebook computers. However, such a TFT LCD has an angular dependence problem.
In order to solve the angular dependence problem, a twisted nematic LCD having an optical compensation plate and a multi-domain LCD has been introduced. In this type of LCD, however, since contrast ratio is dependent upon the viewing angle direction, the color of the image changes.
For a wide viewing angle, an in-plane switching LCD has been proposed in such papers as JAPAN DISPLAY 92 p547, Japanese Patent Unexamined Publication No. 7-36058, Japanese Patent Unexamined Publication No. 7-225538, and ASIA DISPLAY 95 P707.
FIG. 1 is a plan view of a TFT array substrate of the conventional in-plane switching LCD. As shown in FIG. 1, data electrodes 5 and common electrodes 6, which apply electric field to the liquid crystal layer, are arranged in the pixel region defined by a gate bus line 1 and a data bus line 2 crossing each other. At the crossing region of the gate bus line 1 and the data bus line 2, the thin film transistor of which a gate electrode 3 and source/drain electrodes 4 are respectively connected to the gate bus line 1 and the data bus line 2 is positioned. When the voltage is applied to the data electrodes 5 and the common electrodes 6, the electric field is generated in plane of the surface of the substrate. Thus, in the in-plane switching LCD, the liquid crystal molecules are rotated in plane of the surface of the substrate. Thus, the angular dependence problem caused by the refractive anisotropy of the liquid crystal molecule is prevented.
FIG. 2 is a view showing the conventional LCD and FIG. 3 is a sectional view taken along line A--A' of FIG. 2. As shown in the Figures, a metal frame 15 protects the liquid crystal panel 20 except a representation unit 18. In the frame 15, an outer driving circuit including a gate driving circuit 11a and a data driving circuit lib and a back light reflecting plate 13 including a back light 14 are mounted. In the representation unit 18, a light guide plate 29 having a light diffusion plate 21, a polarizer 23, a first substrate 25, a second substrate 26, and an analyzer 24 are sequentially mounted. A liquid crystal layer 2B is formed between the first and second substrates. At that time, the optical compensation plate can be mounted between the polarizer 23 and the first substrate 25 or the second substrate 26 and the analyzer 24.
In general, the elastic coefficient caused by the rotation in the plane level of the in-plane switching LCD is higher than the elastic coefficient caused by the rotation in the vertical direction of the TN LCD, 60 that the response speed of the in-plane switching LCD is lower than that of the TN LCD. In addition, the panel temperature of the conventional in-plane switching LCD is about 30.degree. for the room temperature. At this temperature, the response speed of the liquid crystal having a viscosity of about 20 cp is less than about 70 msec. since a large size LCD more than 20 inches has a slow response speed, i.e., the rotation speed of the liquid crystal molecules, therefore, the image is intermitted and undesirable for dynamic image.